readahead: don't shorten readahead window in read_pages()

Patch series "readahead: Reintroduce fix for improper RA window sizing".

This small patch series reintroduces a fix of readahead window confusion
(and thus read throughput reduction) when page_cache_ra_order() ends up
failing due to folios already present in the page cache.  After thinking
about this for a while I have ended up with a dumb fix that just rechecks
if we have something to read before calling do_page_cache_ra().  This
fixes the problem reported in [1].  I still think it doesn't make much
sense to update readahead window size in read_pages() so patch 1 removes
that but the real fix in patch 2 does not depend on it.

[1] https://lore.kernel.org/all/49648605-d800-4859-be49-624bbe60519d@gmail.com

This patch (of 2):

When ->readahead callback doesn't read all requested pages, read_pages()
shortens the readahead window (ra->size).  However we don't know why pages
were not read and what appropriate window size is.  So don't try to
secondguess the filesystem.  If it needs different readahead window, it
should set it manually similarly as during expansion the filesystem can
use readahead_expand().

Link: https://lkml.kernel.org/r/20241204181016.15273-1-jack@suse.cz
Link: https://lkml.kernel.org/r/20241204181016.15273-2-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/readahead.c b/mm/readahead.c
index e151f4b13ca4bd74e42423fc96dfd24bd4b09f51..50a64d8e06b0ad4bee45ab6fddeb15bc7d929613 100644 (file)
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -158,20 +158,10 @@ static void read_pages(struct readahead_control *rac)
 
        if (aops->readahead) {
                aops->readahead(rac);
-               /*
-                * Clean up the remaining folios.  The sizes in ->ra
-                * may be used to size the next readahead, so make sure
-                * they accurately reflect what happened.
-                */
+               /* Clean up the remaining folios. */
                while ((folio = readahead_folio(rac)) != NULL) {
-                       unsigned long nr = folio_nr_pages(folio);
-
                        folio_get(folio);
-                       rac->ra->size -= nr;
-                       if (rac->ra->async_size >= nr) {
-                               rac->ra->async_size -= nr;
-                               filemap_remove_folio(folio);
-                       }
+                       filemap_remove_folio(folio);
                        folio_unlock(folio);
                        folio_put(folio);
                }